Back in the late 1990s, Ken Caldeira set out to disprove the “ludicrous” idea that we could reverse global warming by filling the sky with chemicals that would partially block the sun. A few years earlier, Mount Pinatubo had erupted in the Philippines, sending tiny sulfate particles — known as aerosols — into the stratosphere, where they reflected sunlight back into space and temporarily cooled the planet. Some scientists believed that an artificial version of this process could be used to cancel out the warming effect of greenhouse gases.

“Our original goal was to show that it was a crazy idea and wouldn’t work,” says Caldeira, who at the time was a climate scientist at the Lawrence Livermore National Laboratory. But when Caldeira and a colleague ran a model to test out this geoengineering scenario, they were shocked…